Steel Transformation During Cooling – Widmanstatten
1. Formation of the Widmanstatten structure
In actual production, hypoeutectoid steel with carbon content (ωc) less than 0.6% and hypereutectoid steel with carbon content greater than 1.2% are cooled by air after casting, hot rolling and forging. The weld seam or heat-affected zone is cooled by air or, when the temperature is very high, rapidly cooled. This results in the growth and precipitation of pre-eutectoid ferrite or pre-eutectoid cementite from the austenite grain boundary along certain needle-shaped crystal planes of the austenite.
2. Microstructure of the Widmanstatten structure
Under a metallographic microscope, one can observe the presence of almost parallel or regularly arranged acicular ferrite or cementite and the pearlite structure between them. This structure is referred to as Widmanstätten, and the following figure illustrates the Widmanstätten of ferrite and cementite.
3. Mechanism of formation of the Widmanstatten structure
Widmanstatten ferrite is formed through a shear mechanism, similar to the bainite process. This results in a convex sample.
Due to the high rate of cooling during formation, ferrite can only precipitate along a specific crystal surface of austenite and has a crystal orientation relationship with its parent phase, austenite.
The formation of acicular ferrite can occur directly from the austenite or the lattice ferrite can be precipitated along the grain boundaries of the austenite and grow parallel in the crystal.
As Widmanstatten ferrite forms, carbon diffuses from the ferrite to the original phase, austenite, on both sides, causing the carbon content of the austenite between the ferrite needles to continually increase and eventually transform into perlite.
Widmanstatten ferrite formed through the bainite transformation mechanism is actually carbon-free bainite.
4. Influencing factors
The formation of the Widmanstatten structure depends on the carbon content, the grain size of the austenite and the cooling rate (transformation temperature) of the steel.
The following figure illustrates the formation temperature and carbon content range for various ferrites and cementites. As can be seen from the figure, the Widmanstatten structure (W zone) can only form under relatively fast cooling rates and within a specific range of carbon content.
For hypoeutectoid steel, if the carbon mass fraction exceeds 0.6%, it becomes difficult to form the Widmanstatten structure due to its high carbon content and low probability of forming a carbon-poor zone.
Research shows that for hypoeutectoid steel, the Widmanstatten structure can only form when the carbon content is within a narrow range of ωc = 0.15% to 0.35% and the cooling rate is rapid, with a fine grain size of austenite.
The finer the austenite grain, the easier it is to form the lattice ferrite, but not the Widmanstatten structure. On the other hand, the coarser the austenite grain, the easier it is to form the Widmanstatten structure and the range of carbon content required to form it becomes wider.
Thus, the Widmanstatten structure is typically observed in steels with coarse austenite grain structure.
5. Properties of the Widmanstatten structure
(1) Widmanstatten is a type of superheated steel structure that can have a negative impact on the mechanical properties of steel. This includes a reduction in impact strength and plasticity, as well as an increase in brittle transition temperature, making the steel more prone to brittle fractures.
(2) It is widely recognized that the impact strength and toughness of steel are significantly reduced only when the austenite grain is coarse, a coarse structure of Widmanstatten ferrite or cementite appears, and the matrix is seriously fragmented.
However, when the austenite grain is relatively fine, even if there is a small amount of acicular ferrite in the Widmanstatten structure present, the mechanical properties of the steel will not be significantly impacted. This is due to the thinner substructure and higher dislocation density of ferrite in the Widmanstatten structure.
(3) The reduction in the mechanical properties of steel due to the Widmanstatten structure is always related to the coarsening of the austenite grains. If the Widmanstatten structure appears in steel or cast steel and reduces its mechanical properties, the first step is to consider whether it is caused by the coarsening of the austenite grain due to high heating temperatures.
(4) For steels prone to Widmanstatten structure, it can be avoided or eliminated by properly controlling the rolling process, reducing the final forging temperature, controlling the cooling rate after forging, or changing the heat treatment process, such as quenching and tempering, normalizing, annealing or isothermal quenching to refine the grain.
6. Valorization of Widmanstatten structure
Steel Transformation During Cooling – Martensite
Structure, structure and properties of martensite crystal
1. Definition
(1) Martensitic Transformation: The non-diffusive phase transformation that occurs when steel is rapidly cooled from the austenitic state to prevent its diffusive decomposition (below the MS point) is known as martensitic transformation.
It is important to note that transformation is characteristic of martensite and the transformation products are all called martensite.
(2) Martensite: In essence, martensite in steel is an interstitial solid solution where carbon is supersaturated in α-Fe.
2. Crystal structure of martensite
The martensitic crystal structure can take the following forms:
- Body-centered cubic: This is the crystal structure of martensite found in low-carbon steels or carbon-free alloys.
- Body-centered tetragonal: This is the crystalline structure of martensite found in steels with a high carbon content.
- Hexagonal lattice: This is the crystal structure of martensite found in complex iron-based alloys at low temperatures.
3. Microstructure of martensite
There are two basic forms of martensite in steel: lath martensite (displacement martensite) and lamellar martensite (also known as needle martensite).
(1) Martensite Lath
Lathed martensite is a common martensitic structure found in low-carbon steel, medium-carbon steel, maraging steel, stainless steel and other iron-based alloys.
a) Structural morphology: martensite laths (D) → martensite beam (B-2; C-1) → group of laths (3-5) → martensite lath.
b) Dense slats are generally separated by residual austenite with high carbon content.
The presence of this thin layer of residual austenite can significantly improve the mechanical properties of the steel.
c) There are a large number of dislocations in lathed martensite and the distribution of these dislocations is not uniform.
It forms a cellular substructure, called a dislocation cell, which is why it is also called dislocation martensite.
(2) Lamellar Martensite
Lamellar martensite is found in high carbon steel (ωC > 0.6%), nickel (ωNi = 30%), stainless steel and some non-ferrous metals and alloys.
Related Reading: Ferrous vs Non-Ferrous Metals
(a) Structural Morphology: The spatial morphology of lamellar martensite is in the shape of a convex lens.
Due to the cutting of the sample during polishing, its cross section appears similar to a needle or a bamboo leaf under the optical microscope.
Therefore, lamellar martensite is also known as needle-shaped martensite or bamboo leaf-shaped martensite.
(b) Microstructural features: The martensite sheets in lamellar martensite are not parallel to each other.
In an austenite grain, the martensite formed by the first sheet generally covers the entire austenite grain and is divided into two parts, causing the size of the martensite sheets formed later to become smaller and smaller.
(c) Size: The maximum size of lamellar martensite depends on the original size of the austenite grain. The larger the austenite grain, the coarser the martensite sheet.
(d) Cryptocrystalline Martensite: When the largest piece of martensite is too small to be distinguished by an optical microscope, it is called “cryptocrystalline martensite”.
Martensite obtained by normal quenching in production is generally in the form of cryptocrystalline martensite.
(e) Substructure: The substructure of lamellar martensite is mainly twinned, which is why it is also called twin martensite.
The twins are typically located in the center of the martensite and do not extend to the edge region of the martensite sheet. The edge region contains high-density dislocations.
In steels with carbon content ωC > 1.4%, a thin twin region with high density can be seen at the centerline of the martensite plate.
(f) Microcracks: The rapid formation of martensite generates a considerable stress field when it collides with other martensite or austenite grain boundaries.
Lamellar martensite is hard and brittle, and the stress cannot be relaxed by sliding or double deformation, making it susceptible to impact cracking.
In general, the larger the austenite grain and the larger the martensite sheet, the more microcracks will form after quenching. The presence of microcracks increases the fragility of steel parts with a high carbon content.
Under the influence of internal stress, microcracks will eventually expand into macrocracks, leading to cracking of the part or a notable reduction in its fatigue life.
(g) Morphology: The morphology of martensite depends mainly on the carbon content of the austenite and is related to the initial temperature of martensite transformation (MS point) of the steel.
The higher the carbon content of the austenite, the lower the MS and MF points will be.
| carbon content | Form | Forming temperature (general) |
| ωC<0.2% | slat martensite | Above 200℃ |
| ωC>0.6% | plate martensite | Below 200℃ |
| ωC=0.2%~1% | Mixed batten and sheet structure | The board knight is formed first and then the piece knight is formed |
(h) Influence of Elements on Martensite Morphology: Elements such as Cr, Mo, Mn and Ni (which decrease the MS point) and Co (which increase the MS point) increase the probability of formation of lamellar martensite.
4. Properties of martensite
(1) Mechanical Properties: Martensite is characterized by high strength and hardness.
(2) Effect of carbon content on properties: The hardness of martensite mainly depends on its carbon content.
When ωC <0.5%, the hardness of martensite increases sharply with increasing carbon content.
However, when ωC > 0.6%, although the hardness of martensite increases, the hardness of steel decreases due to the presence of a greater amount of residual austenite.
(3) Influence of Alloying Elements: Alloying elements have a minimal effect on the hardness of martensite, but can increase its strength.
(4) Hardness: Martensite has varying levels of hardness and strength, which are mainly achieved through solution strengthening, phase transformation strengthening and aging strengthening.
Details are as follows:
Solid solution strengthening: The presence of interstitial atoms in the octahedral gap of the α-phase lattice creates a square distortion in the lattice, which generates a stress field.
This stress field interacts strongly with the dislocations, thus increasing the strength of the martensite.
Strengthening phase transformation: During the transformation into martensite, high-density lattice defects are formed in the crystal. The high-density dislocations in slatted martensite and the twins in lamellar martensite inhibit the movement of the dislocations, thereby strengthening the martensite.
Strengthening by Aging: After the formation of martensite, atoms of carbon and alloy elements diffuse, segregate or precipitate into dislocations or other lattice defects, trapping the dislocations and making it more difficult for the dislocations to move, thus strengthening the martensite.
(5) Strength of martensite: The smaller the size of the martensite lath group or sheet, the greater the strength of martensite. This is because the martensite phase interface prevents dislocation movement, and the smaller the original austenite grain, the greater the strength of the martensite.
The plasticity and toughness of martensite mainly depend on its substructure. Double martensite has high strength but low toughness, while displacement martensite has high strength and good toughness.
(6) Volume of Martensite: Among the various steel structures, austenite has the smallest specific volume and martensite has the largest specific volume.
Thus, the volumetric expansion of steel during quenching is an important factor in generating large internal stresses, deformations and even cracks in the part.
Characteristics of martensite transformation
The driving force behind the martensite transformation, like other solid phase transformations, is the difference in chemical free energy per unit volume between the new phase (martensite) and the original phase (austenite). The resistance to this phase change is also influenced by the interface energy and the strain energy generated during the formation of the new phase.
Despite the presence of a coherent interface between austenite and martensite, the interface energy is small. The large coherent strain energy, caused by the significant difference in specific volume between martensite and austenite and the need to overcome the shear strength and generate numerous lattice defects, leads to the increased elastic strain energy and large transformation resistance of martensite. As a result, sufficient subcooling is required to ensure that the transformation driving force overcomes the transformation resistance, allowing the transformation of austenite to martensite to occur.
The initial temperature of martensite transformation, denoted as “ms”, is defined as the temperature at which the free energy difference between martensite and austenite reaches the minimum driving force required for transformation.
The martensite transformation is a subcooled austenite transformation that occurs at low temperatures.
Compared with pearlite transformation and bainite transformation, martensite transformation has the following distinct characteristics:
- Non-Diffusive Nature of the Martensite Transformation
The transformation of martensite occurs when austenite is subcooled. At this time, the activity of iron atoms, carbon atoms or alloying elements is very low, so the transformation occurs without diffusion. There is only a reconstruction of the network rules and there is no change in composition between the new phase and the parent phase.
- Shear coherence of martensite transformation
Shear refers to deformation caused by two close parallel forces, equal in size and opposite directions, acting on the same object. During martensite transformation, the upper surface of the pre-polished sample tilts and becomes convex, which demonstrates that martensite transformation is directly related to the macroscopic properties of the original phase and that martensite is formed by shear.
Martensite and its parent phase, austenite, remain coherent, with atoms at the interface belonging to martensite and austenite. The phase interface is a shear coherent grain boundary, also known as the habit plane.
Martensite transformation is a phase transformation process in which the new phase is formed on crystalline planes and habits specific to the parent phase and maintains coherence through shearing of the parent phase.
- The transformation of martensite occurs within a temperature range
Martensite Nucleation
Martensite nucleation is not uniform throughout the alloy, but occurs in favorable positions within the original phase, such as lattice defects, regions of deformation, or carbon-poor regions.
Martensitic Transformation Process
Like other phase transitions in the solid state, the transformation of martensite also occurs through nucleation and growth. The transformation is a short-range migration of atoms, and after the formation of a crystalline nucleus, the growth rate is very fast (102 to 106 mm/s) and remains high even at low temperatures.
Martensite transformation rate
The rate of martensite transformation is determined by the nucleation rate and ends when all nuclei larger than the critical nucleation radius are exhausted. The greater the undercooling, the smaller the critical size of nucleation. Additional cooling is required for the smaller nuclei to nucleate and grow into martensite.
For general industrial carbon steel and alloy steels, martensite transformation occurs during continuous cooling (variable temperature). The austenite in the steel is cooled below the MS point at a rate greater than the critical quenching rate, resulting in the immediate formation of some martensite. The transformation has no incubation period and, as the temperature decreases, additional martensite is formed, with the first martensite formed not growing. The martensitic transformation increases as the temperature decreases.
The amount of martensite transformation is determined exclusively by the temperature reached during cooling and is not influenced by the retention time.
Retained Austenite
If the Ms point of high-carbon steel and many alloy steels is above room temperature and the Mf point is below room temperature, a significant amount of untransformed austenite will remain after quenching and cooling to room temperature, known as retained austenite.
To completely transform retained austenite, it can be subjected to “cold treatment,” such as being placed in liquid nitrogen.
Factors that affect the amount of austenite retained include higher carbon content and the presence of elements that reduce DM.
Mechanical Stabilization of Retained Austenite
The mechanical stabilization of austenite refers to the stabilization phenomenon caused by large plastic deformation or compressive stress during quenching. Retained austenite is related to mechanical stabilization. The austenite surrounded by martensite is in a compressed state and unable to transform, leading to its retention.
Deformation-Induced Martensite (Deformed Martensite)
Plastic deformation of austenite above the MS point can result in martensite transformation. The greater the amount of deformation, the greater the amount of martensite transformation. This is referred to as strain-induced martensite transformation.
- Reversibility of the Martensite Transformation
Reversibility refers to the ability of some iron, gold, nickel, and other nonferrous metals to transform austenite to martensite upon cooling and then back to austenite upon reheating without diffusion.
However, this reverse transformation according to the martensite transformation mechanism generally does not occur in carbon steel, as martensite decomposed into ferrite and carbide during heating. This process is known as tempering.
